The source of heat for many homes is hot water and the temperature of that water determines the amount of heat distributed. It is well-known that as the outdoor temperature rises, the temperature of the hot water needed for heating a home can decrease. A heating system can save energy by lowering the indoor water temperature when the outdoor temperature rises.
Heating systems which take advantage of this fact to save energy require an outdoor temperature sensor, a water temperature sensor, and some mechanism to convert the signals from those sensors into a control signal for the hot water heater.
Certain heating systems, in particular the ones described in U.S. Pat. No. 3,604,957 to Satula and U.S. Pat. No. 3,995,810 to Banks, use forward-biased diodes to measure the outdoor temperature and the water temperature. Over a limited temperature range and with constant current, the voltage across a p-n junction varies linearly with temperature. Satula and Banks compare a present voltage level to the sum of the voltages across the temperature sensing diodes. The result of the comparison is used to control the water heater.
Although these and similar apparatuses are in common use, they have several disadvantages which have plagued this type of heating system for a long time. Some of the disadvantages stem from the small voltage signals produced by diode sensors. The change across a p-n junction is only about -2mv/.degree.K for a silicon device. Usually, many diodes must be connected in series to measure temperatures reliably.
The more serious problem which results from the diodes' small voltage signals is that the signals from the sensors are highly susceptible to interference. The low voltage signals at remote sensors can only detected by a very sensitive receiver. Unfortunately, the more sensitive the receiver, the more likely it is to detect noise signals and mistake them for valid signals.
In addition, the outdoor sensors for such systems cannot be placed too remote from the receiver because line losses between the sensor and receiver reduce the magnitude of the sensor signals and they may become undetectable. This problem can be solved by amplifying the signals at the diode sensor or by using the sensor signals to modulate a carrier signal. These solutions, however, significantly increase the price and complexity of the system.
Another disadvantage inherent with diode temperature sensors is that the temperature characteristics of diodes tend to vary substantially. For each diode sensing water temperature controller, the diodes must be carefully chosen. Their temperature characteristics must not only match each other, a task which becomes increasingly difficult as more diodes are added to increase the sensor signal, but also the diodes must match the characteristics of the receiver.
After the diodes are matched, resistance values in the system must be calculated and resistors individually selected depending, among other things, on the diodes' characteristics and the distance between the diodes and the receivers.
Since this procedure must be followed for each water temperature controller manufactured, the production of controllers using diode temperature sensors is slow and expensive.